This invention generally relates wall sheathing, particularly relates to wall sheathing for the design and construction of wind and seismic force-resisting systems used in building structures.
In order to protect a structure such as a building and the people who may occupy or be in the vicinity of such structures, seismic force-resisting systems have been devised. The International Building Code (IBC) defines many of these systems, one being “light-framed walls sheathed with wood structural panels rated for shear resistance or steel sheets.” Light-framed walls are constructed with wood studs or cold-formed steel studs spaced at 16″ or 24″ on center.
Wood structural panels rated for shear resistance include “plywood” and “OSB” (oriented strand board) which are generic products manufactured by many companies. Sheet metal is also a generic product manufactured by many companies. In addition to these two generic wall sheathing products, there is a wall sheathing product called Sure-Board® by Intermat Inc. While plywood is commonly used with both wood studs and cold-formed steel studs, Sure-Board® is used only with cold-formed steel studs. These wall sheathing products are discussed in more detail below.
The ability of a product or system to dissipate energy can be measured by calculating the area of its hysteresis loop. In structural engineering, hysteresis refers to the path-dependence of a structure's restoring force versus deformation. A hysteresis loop is a graph plotting the force versus deflection data obtained by physically testing a product or system through multiple push-pull cycles. The larger the area of the loop, the more energy dissipated. The area of the loop is dependent on its basic shape (pinched or full) and its length (short or long). For the purposes of this disclosure, the term “pinched” describes the shape of a hysteresis loop that appears to be pinched in the middle, as shown in FIG. 1, compared to a hysteresis loop which is “full,” as shown in FIG. 2. A full hysteresis loop indicates the product or system will dissipate more energy than a product or system with a pinched hysteresis loop. For the purposes of this disclosure, the term “short” describes the shape of a hysteresis loop that appears to be short as a measurement of its length, as shown in FIG. 1, compared to a hysteresis loop that is “long,” as shown in FIG. 2. A full and long hysteresis loop indicates the product or system will dissipate more energy than a product or system with a pinched and short hysteresis loop. Also, as the displacement capacity of the product or system increases (length of hysteresis loop), its ductility increases. Ductility is defined as a measure of a material's ability to undergo appreciable plastic deformation before fracture. A common way to characterize the displacement capacity of a system or product is by its drift ratio. Drift ratio is calculated by dividing the horizontal displacement taken from the hysteresis loop graph by the height of the specimen tested. This ratio provides a means to compare specimens of varying heights.
Plywood panels are the most common wall sheathing in use today. These panels were introduced in the 1930's and have undergone extensive testing. Plywood panels, which are typically manufactured in 4′×8′ sheets, are fastened to wood studs with nails and to cold-formed steel studs with self-tapping screws. Plywood panels are typically much stronger and stiffer than the fastener and/or substrate they are fastened to. This being the case, when subjected to horizontal shear forces induced by wind or earthquake, walls sheathed with plywood panels tend to rack rather than deform, resulting in the nails bending and pulling out of wood studs or the self-tapping screws gouging an elongated hole in the cold-formed steel studs. Both of these general failure modes result in limited displacement capacity and limited energy dissipation of the wall panel. A typical hysteresis loop for plywood sheathing fastened to wood studs is illustrated in FIG. 3 and a typical hysteresis loop for plywood sheathing fastened to cold-formed steel studs is illustrated in FIG. 4. Both of these hysteresis loops are pinched and have limited displacement capacity.
Sheet metal sheathing is used exclusively with cold-formed steel studs and is typically fastened to the studs using self-tapping screws. It should be noted that the sheet metal sheathing is typically flat. While plywood panels are very rigid, sheet metals are relatively flexible and rely on their tensile capacity to provide strength and stiffness. The primary failure mode of the sheet metal sheathing is gouging and tearing of the sheet steel at the fastener when subjected to tension loads and buckling when subjected to compression loads. This failure mode results in limited displacement capacity and limited energy dissipation of the wall panel. A typical hysteresis loop for sheet metal sheathing is illustrated in FIG. 5. Similar to plywood sheathing, the sheet metal sheathing has a pinched hysteresis loop and therefore limited displacement capacity.
As mentioned above, an alternative to plywood and sheet metal sheathing products is Sure-Board®. Sure-Board® consists of a flat metal sheet fastened to a rigid board with an adhesive to form a sheathing panel. By varying the type of rigid board the sheet metal is fastened to, Sure-Board® offers a product line to meet various field conditions. Sure-Board® is typically manufactured in 4′×8′ sheets that are fastened to cold-formed steel studs with self-tapping screws. No information is available on the failure mode of the Sure-Board® panels. However, a review of a typical hysteresis loop for a Sure-Board® test panel as illustrated in FIG. 6 shows the same pinched hysteresis loop and limited displacement capacity and limited energy dissipation as plywood and sheet metal sheathing—implying failure occurred at the fastener.
Common to the above noted wall sheathing products is their relative strength and stiffness when compared to the connection that fastens the sheathing to the studs. Typically, panel failure occurs at the connection point, resulting in limited displacement capacity and limited ability to dissipate energy when subjected to external excitations such as wind or earthquake. Accordingly, wall sheathing that resists external excitations such as wind or earthquake of light-framed wall structures with increased displacement capacity and increased energy dissipation is needed.